As the above-described viewing area estimation device, a publicly known viewing area estimation device estimates a viewing area viewed by a monitored subject by detecting positions of the eyeballs of the monitored subject using a camera (see patent literature 1).
However, the viewing area estimation device may not be able to detect an eyeball position when an image capturing condition is affected by a light source, such as sunlight or an illumination or an accessory, such as eyeglasses worn by the monitored subject. In such a case, estimation of a viewing area becomes difficult.